How to use the due date calculator
- Enter the first day of your last menstrual period (LMP).
- Enter your average cycle length in days — use 28 if you're unsure, since that's the standard assumption.
- Press Calculate to see your estimated due date, estimated conception date, and — if the date is in the past — your current gestational age and trimester.
If your cycles are irregular or longer/shorter than 28 days, a dating ultrasound between weeks 8–12 will give a more accurate due date than any calendar-based estimate, including this one.
How due dates are calculated
This calculator uses Naegele's rule, the standard method used in prenatal care: take the first day of the last menstrual period, add one year, subtract three months, and add seven days — mathematically equivalent to adding 280 days (40 weeks). It assumes ovulation occurs around day 14 of a 28-day cycle, so the calculator adjusts slightly if you enter a different average cycle length.
Gestational age vs. fetal age
Doctors track pregnancy in gestational age, counted from the last menstrual period — about two weeks before conception actually occurred. Fetal age, sometimes called conceptional age, counts from the actual conception date instead. The two-week gap is a common source of confusion, especially when comparing due date estimates between a calendar-based calculator like this one and an early pregnancy dating scan.
Trimester breakdown
| Trimester | Weeks | What's typically happening |
|---|---|---|
| First | 1–13 | Major organ development; morning sickness and fatigue are most common here. |
| Second | 14–27 | Often called the most comfortable stretch; anatomy scan usually falls around week 18–20. |
| Third | 28–birth | Rapid weight gain and final organ maturation; more frequent prenatal visits begin. |
Terms related to due date prediction
- LMP
- Last menstrual period — the first day of a person's most recent period before pregnancy, the standard reference point for dating a pregnancy.
- Naegele's rule
- The standard formula (LMP + 280 days) used to estimate a due date, named after the German obstetrician who popularized it.
- Gestational age
- Pregnancy duration measured from the last menstrual period, the convention used on prenatal charts and ultrasounds.
- Dating scan
- An early ultrasound, typically weeks 8–12, used to confirm or adjust the estimated due date based on fetal measurements rather than menstrual dates.
- Full term
- Babies born between 39 and 40 weeks 6 days are considered full term; earlier or later ranges have their own medical classifications.
Why your actual delivery date may differ
- Irregular cycles shift the true ovulation date away from the day-14 assumption.
- Only a small share of babies — around 4–5% — arrive on their exact calculated due date.
- Ultrasound dating in early pregnancy is generally more accurate than LMP-based dating alone, especially for irregular cycles.
Frequently asked questions
The standard method (Naegele's rule) adds 280 days — 40 weeks — to the first day of your last menstrual period, assuming a typical 28-day cycle.
No — it's an estimate. Only around 4–5% of babies arrive on the exact calculated date; most arrive within a window of roughly two weeks either side.
Shorter or longer cycles shift ovulation, so your actual due date can differ from the calendar estimate. A dating ultrasound around 8–12 weeks is more precise for irregular cycles.
Gestational age counts from the last menstrual period, about two weeks before conception actually happened. Fetal age counts from conception itself. Doctors use gestational age by convention.
First trimester: weeks 1–13. Second trimester: weeks 14–27. Third trimester: week 28 until birth.
No — it only does date math from information you provide. A pregnancy test or a doctor's visit is needed to confirm and date a pregnancy accurately.